1. Field of Invention
The invention relates to ballasts for gas discharge lamps, particularly of a kind comprising built-in sensor means operative to sense lamp light output and to controllably maintain it at any desired level.
2. Elements of Prior Art
It is well known that significant improvements in overall cost-effectivity of the lighting function can result from appropriately controlling the level of light output from lighting fixtures used for general lighting in offices and the like.
Fluorescent lamp ballasting systems adapted to permit control of light output level on a systems basis presently do exist--as for instance in accordance with U.S. Pat. Nos. 4,207,498 and 4,350,935 to Spira et al.
However, there are significant complexities associated with practical applications of such light level control systems; and, in spite of the very significant improvements potentially available in overall lighting efficacy, such light control systems have not gained wide acceptance.
3. Inventive Rationale
Much of the value available from a light control system may be attained by control of each individual lighting fixture. That way, for instance, light output from each fixture could be kept constant irrespective of any variations in the magnitude of the power line voltage and/or regardless of changes in luminous efficacy of the fluorescent lamp(s) used in the fixture.
To make this kind of approach commercially feasible, the present invention provides for a ballast having a built-in light sensing means which is so positioned and arranged that, when this ballast is built into a lighting fixture, its light sensor intercepts a proportional part of the light within the fixture and then causes the ballast power output to be so controlled as to maintain the light within the fixture at a desired level.